The present invention relates to an internal-pressure-bearing female screw for use at a location where load variation is great, for instance, in a hydraulic instrument such as an accumulator or a hydraulic cylinder, and more particularly to an internal-pressure-bearing female screw having an excellent fatigue characteristic.
For instance, in an accumulator operating as a hydraulic instrument, an interior of a vessel main body is partitioned by a bladder into a gas chamber and a liquid chamber, the both end portions of the vessel are closed by side plates, also the bladder is allowed to expand and contract in response to hydraulic pressure variation in a hydraulic circuit to make the accumulator perform a pulsation absorbing action and a shock absorber action, and as means for fixing this vessel main body with the side plate, a parallel screw is employed.
Now, if the pressure within the accumulator rises and the side plates are pushed outwards, then to the screw are applied loads in the axial direction and in the circumferential direction, that is, the so-called variable composite load, and this load is not uniformly borne by the respective screw threads but it is greatly deviated in the direction of a tensile force.
Consequently, concentration of stress would occur at the bottom of a valley in the tip end portion of a female screw that is subjected to a large tensile load, and so, the female screw would be ruptured at that position.
Therefore, in order to resolve this problem, it can be conceived to utilize "Screwed Connection Having Excellent Fatigue Characteristics Making Use Of Tapered Male Screw" (See U.S. Pat. No. 4189975 and Japanese Patent Publication No. 56-53651 (1981)) [The title of the U.S. patent is "Screwed Connection Having Improved Fatique Strength."].
The inventor of this invention manufactured a testing accumulator in which a female screw 2 of a vessel main body 1 and a male screw 4 of a side plate 3 are formed of 60-degree triangular screw threads M106.8.times.2 and the thread heights h of the screw threads m.sub.7 - m.sub.1 of this male screw 4 are successively reduced as shown in FIG. 6, also manufactured an accumulator of the prior art type in which the above-described triangular screw threads are used under a standard condition, and load sharing proportions and fatigue lives of the respective screw threads in the respective accumulators were investigated under the condition of a seal diameter d=104 mm, an internal pressure p=0-318 kg/cm.sup.2 and a frequency of 2.5 Hz.
According to the results of investigation, the load sharing proportions were made more even in the case of the testing accumulator than in the case of the accumulator of the prior art type, but the fatigue lives were shorter in the case of the testing accumulator than in the case of the accumulator of the prior art type.
In this connection, a screw thread having the largest load sharing proportion was the second screw thread m.sub.2 counting from the tip end portion 2m and the proportion was 18.5% in the case of the testing accumulator, while in the case of the accumulator of the prior art type such screw thread was the first screw thread counting from the tip end portion and the proportion was 21%, and further, a fatigue life was 560,000 times in the case of the accumulator of the prior art type, while it was 380,000 times in the case of the testing accumulator.
Though it is a common practice that if a load sharing proportion of a screw thread is lowered, a fatigue life of a screw would be extended, in the case of the above-described testing accumulator, on the contrary, the fatigue life was shortened.
Hence, the cause was investigated and it was proved that among the maximum bending moments acting upon the bottoms f of valleys of female screw threads, a peak bending moment is applied to the second bottom f.sub.2 of valley at the tip end portion 2e of the female screw, resulting in the largest amplitude of a bending moment, and failure would arise starting from that location.
In other words, when the female screw 2 is pushed by the male screw 4, the respective screw threads fm of the female screw take the state of a cantilever having a shared load applied onto its lower surface, and so, the thread height fh of the female screw thread fm can be deemed as a span which influences a magnitude of a bending moment.
Therefore, in the case where the thread heights fh are uniform, if the shared loads of the respective screw threads are not made uniform, then the larger the shared load is, the larger becomes the peak bending moment, hence a bending moment amplitude accompanying load variation would become large, and the screw thread becomes liable to be ruptured.
So, when bending moments per unit length arising at the bottoms of valleys of a female screw in the accumulator of the prior art type and the testing accumulator were calculated on the basis of a load, a load sharing proportion and a screw thread contact height, and the results of calculation were as shown in FIG. 3. In this figure, curve A represents the data of the accumulator of the prior art type, while curve B represents the data of the testing accumulator, and from these data it has been proved that a peak bending moment per unit length PB of 13.5 kg mm/mm arises at the bottom of the second female screw thread valley f.sub.2 counting from the tip end portion 2 in the testing accumulator B, furthermore this moment PB is larger than the peak bending moment per unit length PA of 11.4 kg mm/mm in the accumulator of the prior art type A, and consequently, a bending moment amplitude becomes so large that the female screw is liable to be subjected to fatigue failure.
Hence, in order to prevent fatigue failure, it may be conceived to thickne the wall of the female screw or to elongate the screwed length. However, if these countermeasures are employed in a female screw of an accumulator, the vessel main body would become large-sized and becomes unable to be used at a location of small space, and moreover, materials would become wasteful.